Mixture, use of this mixture and process for conditioning a slag located on a metal melt in a metallurgical vessel in iron and steel metallurgy

ABSTRACT

The invention relates to a mixture comprising magnesium, carbon and aluminium to be introduced into the slag located on a metal melt in iron and steel metallurgy, to the use of such a mixture and also to a process for conditioning a slag located on a metal melt in a metallurgical vessel, for example, in a converter, in an electric arc furnace or in a ladle, in iron and steel metallurgy.

The invention relates to a mixture comprising magnesium, carbon and aluminium to be introduced into the slag located on a metal melt in iron and steel metallurgy, the use of such a mixture, and a method for conditioning a slag located on a metal melt in a metallurgical vessel, for example in a converter, in an electric arc furnace, or in a ladle, in iron and steel metallurgy.

In steel and iron metallurgy the pig iron melt is separated from undesired constituents before casting.

If a converter is used then in the case of the LD method, which is currently the most widespread, oxygen is blown for this purpose by means of a lance onto the pig iron melt located in a converter lined with an alkaline refractory material. The process of this blowing of oxygen onto the pig iron melt is also referred to as refining. During refining, iron contaminants, in particular iron contaminants in the form of carbon, manganese, silicon and phosphorous, are oxidised by the oxygen blown in and together with added burnt lime form a slag layer floating on the metal melt.

In an electric arc furnace the crude steel melt is produced by melting scrap metal, pig iron, liquid iron and/or direct reduced iron and further raw materials.

Once the metal melt refined in the primary metallurgical apparatus has the desired properties, this is tapped through the tapping channel into the ladle for secondary metallurgical treatment.

The slag must be selectively influenced or conditioned with regard to chemical and physical properties.

In order to condition the slag it is known to provide the slag with what are known as slag conditioners in order to be able to change the properties of the slag.

The basicity, i.e. the mass or molar ratio of the alkaline components to the other components of the slag (which for example can be calculated in accordance with the following formula: [xCaO+MgO]/[xSiO₂+Al₂O₃+further components]), of the initially acidic or non-alkaline slag must therefore be increased in order to reduce the corrosive attack of the slag on the alkaline lining of the metallurgical vessel in which the metal melt is located and thus attenuate the erosion of the lining and increase the service life thereof. For this reason, slag conditioners have a component increasing the basicity of the slag, this component in particular being lime, dolomite lime, or dolomite.

It is additionally expedient to adjust the content of MgO in the slag by addition of a slag conditioner, such that it lies in the range of saturation of MgO in the slag and a corrosive attack of the slag on the lining is thus attenuated.

It may also be desirable to adjust the viscosity of the slag by the slag conditioner. It is often desirable for the viscosity of the slag during the refinement to be as low as possible such that the iron constituents oxidised by the applied oxygen can be incorporated well into the slag. Furthermore, it may be desirable when tapping or following tapping for the slag to have a high viscosity such that the slag remaining in the converter following the tapping can be better applied to the refractory lining of the converter. Due to this applied slag layer, a corrosive attack of a metal melt on the lining of the converter can be reduced. The process of applying the slag to the converter is also referred to as “maintaining” the converter. The known methods for maintaining the converter on the one hand include what is known as “slag washing”, in which the slag is distributed over the tapping and charging side by pivoting the converter. A further maintaining method is what is known as “slag splashing”, in which the slag is mechanically sprayed with the aid of a nitrogen gas flow of a lance. Lastly, in the case of what is known as “slag foaming”, slag is chemically foamed by adding a carbon carrier. The slag foamed in the case of slag foaming is also referred to as “foamed slag”.

Besides the maintaining of the converter by means of the foamed slag, this also has further advantageous effects. The foamed slag has insulating properties, such that the heat losses from the melt can be attenuated and energy can be saved. Furthermore, components of the metallurgical vessel in which the iron melt is located may be protected by the foamed slag against heat radiation.

In order to generate a foamed slag in an electric arc furnace, carbon blown into the slag is additionally burned by means of oxygen to form carbon monoxide, and the carbon monoxide gas necessary for foaming is provided in this way. In the case of the melting process in an electric arc furnace, a foaming of the slag layer is of importance, since this shields the light arc by means of a volume increase, attenuates radiation losses at the furnace wall, improves the energy transfer to the melt, and thus likewise saves energy.

On this basis, one object of the invention is to provide a slag conditioner by means of which the basicity and the MgO content of the slag can be quickly increased in order to be able to reduce the attack of the slag on the refractory lining of the metallurgical vessel in which the metal melt is located with the slag located thereon.

A further object of the invention lies in providing a slag conditioner by means of which the viscosity of the slag can be selectively adjusted.

A further object of the invention lies in providing a slag conditioner by means of which a foaming of the slag can be achieved.

Lastly, a further object of the invention lies in providing a slag conditioner by means of which an increase of the iron output of the primary metallurgical process can be achieved.

In order to achieve these objects, a mixture or a slag conditioner to be introduced into the slag located on a metal melt in iron and steel metallurgy is provided in accordance with the invention, wherein the mixture comprises magnesium, carbon and aluminium in the following mass proportions:

MgO: 45-90 mass %;

C: 5-40 mass %; and

Al₂O₃: 1-20 mass %.

The mixture according to the invention or the slag conditioner according to the invention is suitable to be introduced into slags on metal melts in any metallurgical vessel, but in particular for slags in converters, electric arc furnaces and ladles.

All of the values specified herein in % are values in mass %, in each case in relation to the total mass of the mixture according to the invention.

The proportions of magnesium and aluminium in the mixture according to the invention are specified as proportions of the oxides thereof Mgo and Al₂O₃ in the mixture, as is usual in refractory technology. However, as described herein, magnesium and in particular aluminium may also be present in the mixture according to the invention in a form other than the oxide form, for example in metallic form or, with regard to aluminium, in the form of carbide.

The MgO saturation of the slag is achieved more quickly by the proportion of MgO in the mixture according to the invention, such that the corrosive attack of the slag on the refractory lining of the metallurgical vessel holding the metal melt is reduced. Furthermore, the viscosity of the slag increases with rising MgO content.

Magnesium is present in the mixture according to the invention preferably as oxide, i.e. in the form of MgO. The proportions of magnesium in the mixture according to the invention are preferably present exclusively in the form of MgO, particularly preferably in the form of sintered or fused magnesia.

MgO may be present in the mixture according to the invention in proportions of at least 45 mass %, i.e. for example also in proportions of at least 48, 50, 52, 54, 56, 57, 58, 59, 60 or 61 mass %. Furthermore, MgO may be present in the mixture in proportions of at most 90 mass %, i.e. for example also in proportions of at most 88, 86, 84, 82, 80, 78, 76, 74, 72, 70, 69, 68, 67, 66, 65, 64 or 63 mass %.

The proportion of carbon in the mixture according to the invention, when the mixture is added to the slag, reacts with oxygen located in the slag to form carbon oxides, in particular to form carbon monoxide CO and carbon dioxide CO₂. When the mixture is introduced into the slag, the carbon in the mixture oxidises immediately and fiercely with oxygen proportions of the slag, such that the slag foams up spontaneously when the mixture is introduced. The slag thus rises in height, as with slag foaming, and covers the refractory lining of the metallurgical vessel. In an electric arc furnace the radiation of the electric arcs is shielded partially or completely with respect to the furnace wall as a result of the increased volume of the foamed slag. Due to the increased content of MgO, the slag at the same time obtains the necessary viscosity in order to also remain adhered to the wall during and after the foaming.

If the mixture comes into direct contact with the metal melt, for example because it arrives through a rinser at an opening in the slag layer, the carbon in the mixture may react directly with oxygen of the metal melt and may remove oxygen from the metal melt. This oxygen removed from the metal melt must not later be removed to a greater extent from the metal melt in additional steps by deoxidising agents, for example aluminium.

At least some of the oxygen with which the carbon from the mixture according to the invention introduced into the slag reacts originates from iron oxides in the slag that are reduced by the carbon to form metallic iron. In contrast to metallic iron, however, iron oxides are fluxing agents, which reduce the viscosity of the slag. Since the proportion of iron oxides in the slag is reduced by the edition of the mixture, the viscosity of the slag can thus be increased. The output of recovered iron in the overall process is also increased.

Due to the proportion of carbon in the mixture, a foaming of the slag can thus be achieved on the one hand. In addition, the viscosity of the slag can be increased. The extent of the foaming of the slag and also the viscosity thereof can thus be set selectively by the proportion of carbon in the mixture.

The carbon may be present in the mixture substantially in pure form, for example in the form of graphite or coke, but for example also combined with further constituents, for example with aluminium proportions or magnesium proportions of the mixture. In particular, it may be that the proportions according to the invention of carbon in the mixture are present partially, largely, or also completely in the form of aluminium carbide (Al₄C₃).

Carbon is present in the mixture according to the invention in proportions of at least 5 mass %, i.e. for example also in proportions of at least 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 or 23 mass %. Furthermore, carbon is present in the mixture according to the invention in proportions of at most 40 mass %, i.e. for example also in proportions of at most 38, 36, 34, 32, 31, 30, 29, 28, 27, 26 or 25 mass %.

Aluminium, calculated as Al₂O₃, may be present in the mixture in a proportion of at least 1 mass %, i.e. for example also in a proportion of at least 2, 3, 4 or 5 mass %. Furthermore, aluminium, calculated as Al₂O₃, may be present in the mixture in proportions of at most 20 mass %, i.e. for example also in proportions of at most 18, 16, 14, 13, 12, 11, 10, 9, 8 or 7 mass %.

The proportion of aluminium in the mixture according to the invention, as described before, is calculated herein as Al₂O₃, wherein, however, the proportions according to the invention of aluminium in the mixture preferably are not present in oxide form as Al₂O₃, but are preferably present partially, largely, or also completely in metallic form and/or in the form of carbide, i.e. as Al₄C₃.

If aluminium is present in the mixture as carbide, this aluminium carbide at the same time forms a carrier both of the proportion of aluminium and of carbon in the mixture.

If carbon and aluminium are present in the mixture in the form of aluminium carbide, the aluminium carbide component is particularly advantageous inasmuch as both the aluminium and the carbon of the aluminium carbide can react with oxygen proportions in the slag, and oxidic constituents of the slag, in particular iron oxides, can be reduced as a result. With corresponding reactions the aluminium proportion of the aluminium carbide oxidises into Al₂O₃ and the carbon proportion of the aluminium carbide oxidises into CO₂.

Insofar as slag conditioners according to the prior art comprise magnesium proportions, these are regularly present in the form of magnesium carbonate (MgCO₃), dolomite, or sometimes also in the form of magnesium hydroxide (Mg(OH)₂). In this respect it is considered advantageous in accordance with the prior art, in the event of contact of these components of the slag conditioners in question with the slag, for the magnesium carbonate to be cleaved into magnesium oxide and carbon dioxide, for the dolomite to be cleaved into magnesium oxide and calcium oxide and also carbon dioxide, and for the magnesium hydroxide to be cleaved into magnesium oxide and steam. Here, the carbon dioxide and the steam cause a foaming of the slag.

It has been found, however, in accordance with the invention that magnesium present in the form of magnesium carbonate, dolomite or magnesium hydroxide leads only to a delayed increase of the basicity and of the MgO content of the slag. Furthermore, it has been found in accordance with the invention that the basicity and the MgO content of the slag can be increased much more quickly and effectively by introducing magnesium in the form of magnesium oxide into the slag. In this respect the mixture according to the invention is produced, in contrast with the prior art, in such a way that the component comprising magnesium is provided in particular in the form of MgO and merely to increase the basicity and MgO content in the mixture, whereas the foaming of the slag is caused by other components in the mixture, in particular by the components comprising carbon and aluminium. Since, due to the slag conditioner according to the invention, no further carbonates have to be introduced either into the primary metallurgical process, the resource efficiency is higher, i.e. the specific consumption and the total weight of slag conditioner to be introduced into the slag and to be transported is lower than in the prior art. In addition, the emissions of carbon dioxide can be reduced by the slag conditioner according to the invention if carbonate-containing slag formers are replaced by the slag former according to the invention.

In accordance with the invention the mixture may contain a proportion of magnesium carbonate less than 10 mass %, i.e. for example also a proportion less than 9, 8, 7, 6, 5, 4, 3, 2, 1 or 0.5 mass %.

Furthermore, the mixture may contain a proportion of Mg(OH)₂ less than 10 mass %, i.e. for example also a proportion less than 9, 8, 7, 6, 5, 4, 3, 2, 1 or 0.5 mass %.

Furthermore, the mixture may contain a proportion of dolomite, in particular of raw dolomite, less than 10 mass %, i.e. for example also a proportion less than 9, 8, 7, 6, 5, 4, 3, 2, 1 or 0.5 mass %.

Furthermore, the mixture may contain a proportion of calcium carbonate or of limestone less than 10 mass %, i.e. for example also a proportion less than 9, 8, 7, 6, 5, 4, 3, 2, 1 or 0.5 mass %.

The mixture is preferably present in a relatively low grain size, for example % in a grain size less than 0.5 mm to an extent of at least 70 mass %, 80 mass %, or at least 90 mass %, or also 100 mass.

By way of example, the grain size of the components of the mixture according to the invention below the grain sizes specified hereinafter may be present in the respective specified mass proportions, wherein the mixture according to the invention for example also may fulfil only one of the following conditions in respect of its grain size:

-   <1 mm: 100 mass %; -   <500 μm 100 mass %; -   <315 μm: at least 90 or 95 mass % and at most 100 mass %; -   <200 μm: at least 85 or 90 mass % and at most 95 or 100 mass %; -   <100 μm: at least 65 or 70 mass % and at most 75 or 80 mass %; -   <63 μm: at least 45 or 50 mass % and at most 65 or 70 mass %.

Since the mixture according to the invention has this very small mean grain size, a particularly good and uniform distribution and particularly also a quick dissolution of the mixture in a slag can be achieved.

In order to be able to achieve good handling of the mixture according to the invention in spite of this small grain size of the mixture, the mixture may be provided in compacted or pressed form, for example in the form of pellets. In order to provide the mixture in the form of pellets, a mixture according to the invention, which in particular may have the previously described grain size distribution, can be pressed into pellets without addition of additives. By way of example, these pellets may have an almond-shaped, rod-shaped or spherical form, for example with a maximum length of for example 50 mm, 40 mm, or 30 mm. The pellets may also have a minimum diameter for example of 5, 10, 15, 20, or 25 mm. Pellets with a corresponding size can be easily handled, but at the same time are still small enough that they quickly disintegrate in a slag following addition thereto, and the advantages of the small grain size distribution according to the invention can materialise there quickly.

It may be that the mixture according to the invention contains a proportion of calcium oxide (CaO), since the basicity of the slag can be further increased thereby and the attack of the slag on the refractory lining of the metallurgical vessel can be lowered. The CaO of the mixture in particular then has an advantageous basicity-reducing effect when the ratio of CaO to SiO₂ in the mixture does not exceed a certain measure.

It has been found in accordance with the invention that the basicity of the slag can be increased in particular by the CaO when the ratio of mass proportions of CaO to SiO₂ in the mixture is not below 0.7. Thus, the ratio of the mass proportions of CaO to SiO₂ in the mixture according to the invention may not lie below 0.7.

SiO₂ may enter the mixture according to the invention fundamentally via impurities of the raw materials of said mixture.

The mixture may comprise calcium oxide and silicon dioxide in the following mass proportions:

CaO: 0 to 10 mass %,

SiO₂: 0 to 7 mass %.

CaO may also be present in the mixture for example in proportions of at least 0.1 or 0.2 or 0.5 or 1 or 1.5 or 2 mass %, and for example in proportions of at most 10, 9, 8, 7, 6, 5, 4, 3 or 2.5 mass %.

SiO₂ may be present in the mixture for example in proportions of at least 0.1 or 0.2 or 0.5 or 1 or 1.5 or 2 mass %, and for example in proportions of at most 7, 6, 5, 4, 3 or 2.5 mass %.

As already discussed, the mixture may be provided in the form of pellets, wherein the mixture is pressed into pellets without the addition of additives. If, however, additives are used in order to press the mixture into pellets, CaO may be used as such a press additive. In this case, in contrast to the previously disclosed inventive concept, in accordance with which the mixture contains proportions of CaO of at most 10 mass %, the mixture may contain proportions of CaO of up to 40 mass %. However, the mixture preferably does not contain any additive for pressing, such that the proportion of CaO in the mixture, as described above, does not lie above 10 mass %.

The mixture may comprise iron oxides in the following mass proportions:

iron oxide: 0 to 7 mass %

Here, iron oxide stands for the sum of all iron oxides in the mixture, i.e. in particular FeO and Fe₂O₃, but for example also Fe₃O₄ and Fe₂O.

Iron oxides may be present in the mixture for example also in proportions of at least 0.1 mass %, 0.2 mass %, 0.4 mass %, 0.6 mass %, or 0.8 mass %, and for example at most in proportions of 7 mass %, 6 mass %, 5 mass %, 4 mass %, 3 mass %, 2.8 mass %, 2.6 mass %, 2.4 mass %, 2.2 mass %, or 2 mass %.

It has been found in accordance with the invention that the advantageous effects described herein of the mixture according to the invention as slag conditioner may be adversely influenced by the presence of further components in the mixture.

Thus, besides the aforementioned components, i.e. MgO, C, Al, Al₄C₃, CaO, SiO₂, iron oxides and optionally Al₂O₃, the mixture may also comprise only small proportions of further components, for example in proportions less than 5 mass %, 4 mass %, 3 mass %, 2.5 mass %, 2 mass %, 1.5 mass %, or also less than 1 mass %.

By way of example, the mixture may comprise proportions of the following components below the mass proportions specified hereinafter:

Cr₂O₃: <0.2 mass %;

P₂O₅: <0.2 mass %;

TiO₂: <0.2 mass %;

K₂O+Na₂O <0.5 mass %;

ZrO₂ <0.2 mass %.

It has been found surprisingly in accordance with the invention that magnesia-carbon products that have been used in the steel industry, in particular as inner linings of basic oxygen furnace converters, in electric arc furnaces or in ladles, are suitable in part as raw material for the mixture according to the invention. In this respect, correspondingly recycled magnesia-carbon products can be used partially, largely or exclusively as raw material for the mixture according to the invention. In this respect, the invention also relates to the use of recycled magnesia-carbon products as raw material for the mixture according to the invention and the use of such recycled magnesia-carbon products as slag conditioner according to the invention.

By way of example, besides recycled magnesia-carbon products, at least one of the following further raw materials may also be selected as raw materials for the mixture according to the invention: magnesia (in particular sintered magnesia), carbon (in particular graphite), corundum, or aluminium carbide.

The invention also relates to a method for conditioning a slag located on a metal melt in a metallurgical vessel in iron and steel metallurgy, said method comprising the following steps:

-   -   providing a mixture according to the invention described herein;     -   introducing the mixture into the slag located on the metal melt         in the metallurgical vessel.

As described herein, the mixture can be provided in compacted or pressed form, for example in the form of pellets.

The mixture provided is introduced onto the slag and sinks thereinto, such that it can develop its effect there.

The mixture according to the invention is suitable in principle as a slag conditioner for slags on a metal melt in any metallurgical vessel, for example for metal melts in converters, electric arc furnaces, or ladles. The mixture according to the invention is particularly preferably used as a slag conditioner for slags on metal melts located in a metallurgical vessel with an alkaline lining, i.e. in particular with a lining based on at least one of the following materials: magnesia, magnesia-carbon, doloma or doloma-carbon.

The invention also relates to the use of a mixture according to the invention described herein for conditioning a slag located on a metal melt in a metallurgical vessel in iron and steel metallurgy.

Here, the use may be implemented as disclosed herein.

All of the features of the invention disclosed herein may be combined with one another arbitrarily, individually or in combination.

The invention will be explained in greater detail on the basis of the following practical example.

A mixture comprising magnesium, carbon and aluminium and also further components in the mass proportions according to Table 1 was first provided in the practical example.

TABLE 1 Mass proportions Component [%] MgO 62.6 C 24.6 Al₂O₃ 6.4 CaO 2.4 SiO₂ 2.3 Fe₂O₃ 1.3 Cr₂O₃ 0.05 P₂O₅ 0.08 TiO₂ 0.08 K₂O 0.05 Na₂O 0.08 ZrO₂ 0.06

The carbon was present in the mixture in the form of graphite and aluminium carbide.

Aluminium was present in the mixture in the form of metallic aluminium and in the form of aluminium carbide.

Recycled magnesia-carbon products were used exclusively as raw materials.

The mixture was provided in the form of pressed, almond-shaped pellets without additional additives, with a thickness of approximately 15 mm and a length of approximately 30 mm.

The grain size distribution of the mixture in the pellets is specified in Table 2.

TABLE 2 Grain size Mass proportions [%]  <63 μm 55 <100 μm 72 <200 μm 92 <250 μm 97 <500 μm 100

The mixture was used as a slag conditioner for a slag on a metal melt located in an oxygen converter. Here, the mixture was placed on the slag located on the melt. Due to the placement of the mixture on the slag, the basicity of the slag could be increased. Furthermore, due to the proportions of carbon, aluminium and aluminium carbide in the mixture, it was possible to achieve a foaming of the slag. Lastly, the viscosity of the slag could be adjusted to the desired measure. 

1. A mixture for introducing into the slag located on a metal melt in iron and steel metallurgy, said mixture comprising magnesium, carbon and aluminium in the following mass proportions: MgO: 45 to 90 mass %; C: 12 to 40 mass %; Al₂O₃: 1 to 20 mass %.
 2. The mixture according to claim 1, with a proportion of MgCO₃ of less than 10 mass %.
 3. The mixture according to claim 2, being present in the form of pellets.
 4. The mixture according to claim 3, the granularity of which is present in a grain size less than 0.5 mm to an extent of at least 70 mass %.
 5. The mixture according to claim 1, comprising calcium oxide and silicon dioxide in the following mass proportions: CaO: 0 to 10 mass %; SiO₂: 0 to 7 mass %.
 6. The mixture according to claim 1, comprising iron oxide in the following mass proportions: iron oxide: 0 to 7 mass %.
 7. A method for conditioning a slag located on a metal melt in a metallurgical vessel in iron and steel metallurgy, said method comprising the following steps: providing a mixture, said mixture comprising magnesium, carbon and aluminium in the following mass proportions: MgO: 45 to 90 mass %; C: 12 to 40 mass %; Al₂O₃: 1 to 20 mass %; and introducing the mixture into the slag located on the metal melt in the metallurgical vessel.
 8. (canceled) 